ArmA Methyltransferase in a Monophasic Salmonella entericaIsolate from Food

ABSTRACTThe 16S rRNA methyltransferase ArmA is a worldwide emerging determinant that confers high-level resistance to most clinically relevant aminoglycosides. We report here the identification and characterization of a multidrug-resistant Salmonella entericasubspecies I.4,12:i:− isolate recovered from chicken meat sampled in a supermarket on February 2009 in La Reunion, a French island in the Indian Ocean. Susceptibility testing showed an unusually high-level resistance to gentamicin, as well as to ampicillin, expanded-spectrum cephalosporins and amoxicillin-clavulanate. Molecular analysis of the 16S rRNA methyltransferases revealed presence of the armAgene, together with blaTEM-1, blaCMY-2, and blaCTX-M-3. All of these genes could be transferred en blocthrough conjugation into Escherichia coliat a frequency of 10−5CFU/donor. Replicon typing and S1 pulsed-field gel electrophoresis revealed that the armAgene was borne on an ∼150-kb broad-host-range IncP plasmid, pB1010. To elucidate how armAhad integrated in pB1010, a PCR mapping strategy was developed for Tn1548, the genetic platform for armA.The gene was embedded in a Tn1548-like structure, albeit with a deletion of the macrolide resistance genes, and an IS26was inserted within the melgene. To our knowledge, this is the first report of ArmA methyltransferase in food, showing a novel route of transmission for this resistance determinant. Further surveillance in food-borne bacteria will be crucial to determine the role of food in the spread of 16S rRNA methyltransferase genes worldwide.